In recent years, there has been a remarkable increase in the demand for larger capacity and higher speed in radio communications, and much research has been conducted into methods of further improving the efficient utilization of limited frequency resources. Utilization of the space domain is drawing attention as one such method. Technologies that utilize the space domain include so-called space division multiplexing (SDM) transmission or MIMO (Multi-Input Multi-Output) technologies whereby different data sequences are transmitted to a terminal apparatus using same-time, same-frequency, same-code physical channels, by utilizing spatial orthogonality in a propagation path.
With MIMO technology, different signals transmitted simultaneously from a plurality of transmitting antennas using the same frequency band are received using a receiver equipped with a plurality of receiving antennas. With this system, system capacity is improved by having the receiver eliminate same-frequency interference by applying adaptive signal processing in the space domain, and detecting a transmitted signal.
Information on MIMO technology is disclosed in Non-patent Document 1, for example. Both the transmitter and receiver are equipped with a plurality of antenna elements, and spatial multiplexing transmission can be implemented in a propagation environment with low correlativity of received signals between antennas. In this case, different data sequences are transmitted from a plurality of antennas with which a transmitter is equipped using a same-time, same-frequency, same-code physical channel on an antenna-by-antenna basis, and a receiver demultiplexes and receives different sequences based on received signals at a plurality of antennas with which the receiver is equipped.
Information regarding the demultiplexing and reception method in this case is disclosed in Non-patent Document 2, for example. It is possible to use a means such as ZF (Zero Forcing), MMSE (Minimum Mean Square Error), MLD (Maximum Likelihood Detection), or an interference canceller, for transmission sequences from a plurality of radio transmitting apparatuses. In this way, it is possible to achieve faster radio communication without using M-ary modulation by employing a plurality of spatial modulation channels.
Also, in the case of an environment in which many scatterers are present between a transmitter and receiver under adequate S/N (signal to noise ratio) conditions, equipping the transmitter and the receiver with the same number of antennas makes possible an increase in communication speed proportional to the number of antennas.
Non-patent Document 1: G. J. Foschini, “Layered space-time architecture for wireless communication in a fading environment when using multi-element antennas,” Bell Labs Tech. J., pp. 41-59, Autumn 1996
Non-patent Document 2: John G. Proakis, “Digital Communications Fourth Edition,” Chap. 14, McGrawHill, 2001